1. Field of the Invention
The present invention relates to a process for manufacturing a hot-dip galvanized steel sheet by a nickel pre-coating method.
2. Description of the Related Art
Heretofore, the process for manufacturing a galvanized steel sheet by a nickel pre-coating method is already disclosed, for example, in Japanese Patent Publication Nos. 46-19282 and 63-48923.
In recent years, galvanized steel sheets, when used for building materials, household electric appliances, automobiles, and the like, are required to give superior surface appearance and superior coating adherence as important properties irrespective of their thickness, or whether they are made from hot-rolled sheets or cold-rolled sheets. The galvanized steel sheets manufactured by a nickel pre-coating method as disclosed in the aforementioned Japanese Patent Publication Nos. 46-19282 and 63-48923 give better external appearance and better coating adherence than those manufactured by other conventional galvanizing processes without nickel pre-coating such as a Sendzimir method or a non-oxidative furnace method. However, the nickel pre-coated galvanized steel sheets disclosed by the prior art, due to unsatisfactory heating conditions such as heating temperature and heating time after the nickel coating, are not optimal for securing surface appearance and coating adherence of building materials and household electric appliances, and coating adherence (or anti-powdering property) and corrosion resistance of severly worked portions for automobile application, especially when they are made from thick hot-rolled acid-pickled sheets. Therefore, further improvement of galvanized steel sheets is desired to be improved further.
The inventors of the present invention studied the manufacturing process of the zinc-coated steel sheets by a galvanizing process employing a nickel pre-coating method to improve greatly the external appearance and the coating adherence, and further the coating adherence and the corrosion resistance of a severely worked portion thereof. Consequently, the inventors have found that the external appearance and the coating adherence, and further the coating adherence and the corrosion resistance of a severely worked portion are greatly improved, by conducting the galvanizing treatment under a specific heating condition after nickel pre-coating, in comparison with conventional galvanizing processes employing a nickel coating method.
As an experiment, a zinc-coating layer was prepared according to a conventional method of manufacturing a zinc-coated steel sheet employing a nickel pre-coating method as described in Japanese Patent Publication No. 46-19282, and the structure of the layer was investigated. In the preparation, after a steel sheet was pre-coated with 0.1 g/m.sup.2 of nickel, the nickel pre-coated steel sheet was heated for 8 seconds at 200.degree. C. which is the lowest heating temperature after nickel pre-coating as disclosed in the examples of the above-described patent publication. As the result, non-coating of zinc was caused and coating adherence was insufficient. Therefore, a more suitable heating temperature and a more suitable heating time were expected to be found for the heating treatment after the nickel pre-coating. In another experiment, a nickel pre-coated steel sheet was heated at 550.degree. C. in a furnace as disclosed in Japanese Patent Publication No. 63-48923. As the result, some improvement was observed in comparison with the case of heating at 800.degree. C. according to the Sendzimir method without nickel pre-coating. However, local non-coating of zinc is liable to occur, and the zinc coating adherence was insufficient and the surface appearance tended to become whitish. The cause is considered to be a tollnos. The heating temperature of 550.degree. C. after the nickel pre-coating is still excessively high, and the heating time is excessively long because of furnace heating. Therefore, it is assumed that the nickel coating layer comes to diffuse into the base steel during the heating to form a Ni-Fe solid solution, becoming liable to be oxidized, which impairs coating adherence of zinc, and promotes formation of alloy of the base steel with zinc.
In view of the above investigation, the heating temperature range and the heating time after the nickel pre-coating. are regarded as being important factors. Hence, investigation was made comprehensively regarding the heating conditions. Consequently, it was found that the surface appearance and the coating adherence of the zinc coating are greatly improved if nickel is coated in an amount of 0.2 to 2.0 g/m.sup.2, and the steel sheet is heated, before entering into a zinc bath containing aluminum at a concentration of from 0.1 to 1.0%, to a temperature within the range of from the melting temperature of the bath to 500.degree. C., and further the steel sheet is kept at a temperature of 350.degree. C. or higher for a time of not longer than 15 seconds before reaching the entering temperature for dipping the steel sheet into the molten zinc bath. Further it was found that the coating adherence after severe working and the corrosion resistance of the worked portion are greatly improved if the heating rate after the nickel pre-coating is raised to 30.degree. C./s or more. The resulting zinc coating layer was found to have a layer structure composed of a reaction layer, which consists of a quaternary alloy layer of Fe-Al-Zn-Ni, formed at the interface of the base steel, and a zinc coating layer formed thereon containing a minute amount of aluminum. Further, the Zn-Fe alloy layer on the base steel interface was found to be extremely thin.
As to the manufacturing process for realizing the manufacturing conditions such as rapid heating at a low cost with compact equipment, the beforementioned Japanese Patent Publication No. 46-19282 does not discloses specifically the constitution of the heating apparatus. On the other hand, the beforementioned Japanese Patent Publication No. 63-48923 discloses a heating method employing an indirect heating furnace. The indirect heating furnace, however, is based on radiation heating, employing a refractory-lined furnace body, which is not suitable for rapid heating and requires a long length of the furnace, resulting in a disadvantageous large-scale and expensive heating apparatus.
The inventors of the present invention investigated mainly the heating method, and have found a method for realizing an inexpensive and compact installation and conditions for producing a hot-dip galvanized steel sheet by employing direct resistance heating.
The present invention has been achieved based on the above-described findings.